Syntheses, Crystal Structures, SHG Response and Purple Luminescent Property of Tetra(isothiocyanate) Mn(II) and Substituted Benzyl Triphenylphosphonium Cations①

LI Yu-Kong TAN Yu-Hui LIU Yao TANG Yun-Zhi WEI Wen-Juan SONG Ning DU Peng-Kang HAN Ding-Chong

(Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou 341000, China)

ABSTRACT Researches on multifunctional molecular materials with both nonlinear optical activity and fluorescence properties have received much interest in molecular chemistry since they have wide application prospects. Here a novel ionic compound 2(BPP)+·[Mn(NCS)4]2- (1) was synthesized via the assembly of benzyltriphenylphosphorus chloride, isothiocyanate and manganese chloride, which displays both strong SHG response and purple luminescence. Compound 1 belongs to monoclinic system and crystallizes in Cc space group. X-ray single-crystal diffraction analysis shows that Mn2+ ions are tetra-coordinated with N atoms of four NCS- ions through forming a distorted tetrahedral configuration. The anion Mn(NCS)42- forms a 1D chain structure through strong S···S interaction. At the same time, there are abundant C–H···S and π···π interactions, which further accumulate into a three-dimensional supramolecular structure. Solid-state fluorescence studies show that the complex 1 has strong purple fluorescence with emission wavelength of 396 nm under excitation wavelength 248 nm. In particular,the second harmonic generation (SHG) measurements show that compound 1 has nonlinear optical activity and its SHG response is 2.25 times that of standard potassium dihydrogen phosphate (KDP). This multifunctional molecular material with SHG response and purple fluorescence is of great significance for the development of new metal organic complexes with potential application prospects.

Keywords: benzyltriphenylphonium chloride, crystal structure, SHG responses, luminescent properties;

1 INTRODUCTION

Noncentrosymmetric (NCS) molecular materials have received much interest because of their potential applications in the areas of nonlinear optical (NLO) materials, ferroelectrics,piezoelectric,etc. Among these NCS materials, NLO crystal material with distinct SHG (second harmonic generation)response is one of the important optoelectronic information functional materials, and nonlinear optical elements in modulation switch, remote communication, information processing and other fields show a trend of accelerating development[1].For instance, a NLO crystal material coupling other physical property such as photo-luminescent property, SHG responses ferroelectric,etc. will make them bear multi-function and have many practical applications[2,3].

There is considerable interest in utilizing the crystal engineering techniques and the principles of supramolecular chemistry to prepare molecular materials with specific structural, optical, conductive, and magnetic properties, in which some non-covalent interactions such as weak·p··πorπ···πstacking interactions and H-bonding interactions are employed to obtain some new materials. On one hand, ligands containing heteroaromatic rings with large·π··πstacking interactions construct all kinds of supramolecules. Moreover, it can be an important part of light-emitting materials to effectively enhance the fluorescence response of the compounds[4-7]. On the other hand, inorganic complex anions with Cl-, Br-, CN-and SCN-are proved to be very useful and potential building block for organic-inorganic hybrid materials[8,9].Besides, manganese(II) compounds have attracted great interest due to their interesting structural diversity and potential applications in light-emitting materials. Mn(II) complexes with excellent luminescent properties have attracted extensive attention due to their low cost and low toxicity. They have potential applications in photoluminescence, electroluminescence, luminescent sensors and biomarkers[10]. However, the inorganic-organic hybrid materials containing tetra (isothiocyanate) Mn(II) anion and heteroaromatic rings organic cation are scarcer.

Keep these in mind, we choose benzyltriphenylphosphine and thiocyanate as ligands, assembled with Mn(II) salt in a mixed solution, and obtained a novel compound 2(BPP)+·[Mn(NCS)4]2-. As we expected, compound 1 exhibits strong SHG responses and excellent fluorescence properties. Here we described its crystal structure, luminescent properties and strong SHG response which may be influenced by the change of substituted group on the phenyl ring of benzyl group in the cations, and shows that it is an excellent fluorescent and nonlinear material.

2 EXPERIMENTAL

2. 1 Materials and physical techniques

All reagents and solvents employed in this experiment were obtained from commercial sources and used directly without further purification. PL emission spectra were measured at room temperature on a spectra fluorophotometer (JASCO,FP-6500). At room temperature, Ultraviolet-visible (UV/Vis)diffuse reflectance spectroscopy of 1 was measured by a Shimadzu (Tokyo, Japan) UV-2550 spectrophotometer in a range of 200～800 nm. BaSO4was used as the 100% reflectance reference. The powder crystals of 1 were used for the measurement. The elemental analyses were measured on a Vario EL III elemental analyzer. Thermogravimetric analysis (TGA)measurements were performed on a TA-Instruments STD2960 system from 298 to 1100 K. Powder X-ray diffraction (PXRD) data were recorded on a Rigaku D/MAX 2000 PC X-ray diffraction instrument. The PXRD diffraction was measured with CuKαradiation (λα1= 0.1540598 nm,λα2=0.1544426 nm) under the generator voltage (40 kV) and tube current (40 mA) by using continuous scan type from 5.0 to 50.0° at room temperature.

2. 2 Synthesis of the title complex

Compound 1 was prepared by the conventional solution method (Scheme 1). MnCl2?4H2O (1 mmol, 0.1258 g) and KSCN (4 mmol, 0.3887 g) were stirred in methanol (15 mL)solvent for 45 minutes. The precipitate was filtered out and the clear solution was taken. Then the methanol solution of benzyltriphenylphosphonium chloride (2 mmol, 0.7774 g)was dropped into the clear solution and stirred for 30 minutes.The obtained filtrate was volatilized slowly at room temperature. For 1, yield, 0.01832, 78% based on BPP. Calcd. (%) for 1: C, 65.25; H, 4.43; N, 5.64. Found (%) for 1: C, 65.44; H,4.67; N, 5.55. IR (KBr, cm-1,sfor strong,mmedium,wweak):3410 (W), 3100 (W), 2825 (W), 2050 (s), 1400 (m), 1487(m),1588(m), 1125 (m), 750 (m), 500 (m). As shown in Fig. S2,powder X-ray diffraction (PXRD) patterns were collected at room temperature apparatus, and the fitting results of compound 1 can be well matched, proving that compound 1 is a pure phase.

Scheme 1. Preparation of compound 1

2. 3 Crystal structure determination

Single crystal of 1 was obtained directly from the above preparation. The single-crystal X-ray diffraction studies were performed with a Bruker Smart Apex II single crystal diffractometer operating with a graphite-monochromated Mo-Kαradiation (λ= 0.71073 ?). The crystal was kept at 299.9 K during data collection. Using Olex2[11], the structure was solved with the SHELXS structure solution program by direct methods and refined with the SHELXL refinement package using least-squares minimization[12]. All non-hydrogen atoms except the guest molecules were refined by full-matrix least-squares techniques with anisotropic displacement parameters and the hydrogen atoms were geometrically fixed at the calculated positions attached to their parent atoms, and treated as riding atoms[13]. Crystal data for 1, space groupCcwitha= 11.0435(11),b= 23.857(2),c= 20.012(2) ?,V=5083.0(8) ?3,Z= 4,μ(MoKα) = 0.527 mm-1,C54H44MnN4P2S4,Mr= 994.05,Dc= 1.299 g/cm3,F(000) =2060 andGOOF= 1.038. For 1, 44587 reflections measured(6.40?≤2θ≤55.26?), 11772 unique (Rint= 0.0427,Rsigma=0.0514) which were used in all calculations. The finalR=0.0505 (I> 2?(I)) andwR= 0.1199 (all data). The Flack parameter was 0.00(3) (Fig. 1a). The selected bond lengths and bond angles for 1 are given in Table 1. The hydrogen bond parameters are shown in Table S1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compound 1

3 RESULTS AND DISCUSSION

3. 1 Description of the structure

As shown in Table 1, compound 1 crystallizes in monoclinic system, space groupCc. Each asymmetric unit consists of one[Mn(NCS)4]2-anion and two [BPP]+cations. Fig. 1(a) depicts the coordination environment of the Mn(II) atom with atomic numbering scheme. Every Mn2+ion binds to four N (N(1),N(2), N(3), N(4)) atoms of thiocyanate，and the [Mn(NCS)4]2–anion presents seriously distorted tetrahedral coordination geometry[14,15]. For [Mn(NCS)4]2–anions, the average Mn–N bond length is 2.05 ?, the N–Mn–N bond angles fall in the 105.000～114.500° range, and the N–C–S bond angles of four NCS–groups is averaged by 178.500(6)°. In addition, the average C–N and C–S bond distances are 1.13 and 1.60 ?,respectively, similar to those analogues containing[Mn(NCS)4]2–anions[33-35]. The nearest Mn·· Mn distance between two adjacent anions is 11.34 ?[16]. For each [BPP]+cation, the average C–P bond length is 1.80 ?, and the C–P–C bond angles are in the range of 108.50～113.9°, so the four C atoms together with the P atom form a regular tetrahedron.

Fig. 1. (a) Coordination environment of 1 and (b) the 1D chain of [Mn(NCS)4]2-anions through intermolecular S···S interactions between anions.Symmetry codes: A: x, 1–y, 1/2+z; B: x, y, 1+z; C: x, 1–y, 3/2+z

The most interesting fact is that the [Mn(NCS)4]2-anions form a linear chain through S···S interaction (Fig. 1b), and the S···S distance of 3.412 ? is shorter than the sum ofvan der Waalsradii of two sulfur atoms, indicating a strong supramolecular interaction[17,18]. It is also worth noting that the C–H···S hydrogen bond is found between the S(3) atom of the anion and the adjacent cation (Fig. 2a): C(16B)–H(16B)···S(3A) (symmetry code: A =x, –y, –1/2+z, B: 1+x,y,z) with C(16B)···S(3A) to be 3.488 and H(16B)···S(3A)being 3.254 ?[19,20]. As shown in Fig. 2b, there are also many·π··πstacking interactions between neighboring [BPP]+cations in complex 1, and the distance is 3.554 ?. The [BPP]+cations filled in two chains of [Mn(NCS)4]2-anions through S···S interaction. From the stacking diagram along theaaxis(Fig. S3), it can be seen that the thiocyanate radicals are connected like an irregular hexagon, which encapsulates the organic phosphorus in the cavity and enhances the stability of the structure. Additionally, a comparison between 1 and the[BzTPP]2[Zn(NCS)4] compound previously reported reveals that when the cation is identical[21], while the metal ion of the anion changes from Zn(II) to Mn(II), the crystal system and space group are still the same, but the shortest S···S, M···P and M···M (M = Mn or Zn) distances between the adjacent anions and cations, the stacking mode and the weak interactions of the cations and anions are significantly different[20].

Fig. 2. (a) C–H·· S hydrogen bonds between the S(3) atom of the anion and the adjacent cation and (b) π··π interaction in compound 1

3. 2 Luminescent property and solid-state UV/Vis diffuse reflectance spectroscopy

The photo-luminescent properties of 1 were investigated in solid state at room temperature (Fig. 3). Under 248 nm excitation, 1 shows a strong emission band at 396 nm. As documented, the complex 2(BzTPP)+[Zn(NCS)4]2-has a broad luminescence peak in the range of 327～361 nm, with a maximum of 356 nm[21]. Obviously, when the anion[Zn(NCS)4]2-changes to [Mn(NCS)4]2-, the maximum emission peak shifts to an Einstein shift of 40 nm, indicating that[Mn(NCS)4]2-plays an important role in the fluorescence emission of the compounds, and the magnitude of the peak value is attributed to the ligand metal transition[10,22]. What’s more, the C–H·· S hydrogen bonds between the[Mn(NCS)4]2?anion and the [BPP]+cation improved the emission intensity[21]. Interestingly, 1 also exhibits interesting semiconductor properties. In order to deeply investigate the mechanism, solid-state UV/Vis diffuse reflectance spectroscopy was performed at room temperature. As shown in Fig. 3b, an intense absorption occurs at the band edge onset of 320 nm. The corresponding optical band gap can be defined as 4.0 eV according to Tauce equation (Fig. S4, inset).This value is within the band gap of most known organic-inorganic hybrid perovskites (3.5～4.5 eV)[36].

Fig. 3. (a) Fluorescence spectrum of compound 1 at room temperature. (b) UV/Vis absorption spectrum of 1. The inset shows the Tauc plot, and the estimated band gap is 4.0 eV

3. 3 SHG effect analysis

The compounds with the second-order nonlinear optical properties must be chiral or non-centric[23-28]. Because the title compound adopts space groupCc, we study its nonlinear optical properties[27]. The second harmonic generation (SHG)measurements show that compound 1 has nonlinear optical activity and its SHG response is 2.25 times that of standard potassium dihydrogen phosphate (KDP). The reason can be attributed to the coordination of N from thiocyanate to a metal center, thus resulting in the donation of the lone pair of electrons on the N atoms to the metal center and the formation of an excellent donor acceptor (D-A) system[23,31,32]. Furthermore, the C–H···S hydrogen bonds between the[Mn(NCS)4]2?anion and the [BPP]+cation can effectively mediate the push-pull strength. In addition, the mechanism of SHG formation may be attributed to the formation of a distorted tetrahedron formed by the coordination of thiocyanate with manganese ions, and the polarization effect cannot be counteracted[28], which makes the complex as a potential secondorder nonlinear optical material.

Fig. 4. SHG signals of compound 1

3. 4 Thermal analysis

In order to study the thermal stability of compound 1, thermal analysis (TGA) was carried out in N2(100 mL/min) at a heating rate of 10 °C·min-1from room temperature to 1100 K.As shown in Fig. 5, complex 1 has high thermal stability below 573 K. Above 431 K, the DTA curve of 1 shows an obvious rising peak which may be caused by the high nitrogen content and high energy[31]. When the temperature is higher than 573 K, the compound loses weight, corresponding to the release of organic ligands. The thermal stability of complex 1 is higher than that of similar compounds due to the strong S???S interaction and abundant C–H???S andπ???πinteractions.

Fig. 5. TGA diagram for compound 1

4 CONCLUSION

In conclusion, a new manganese compound 2(BPP)+·[Mn(NCS)4]2-(1) was synthesized by the simple solution method. It has strong purple luminescent property with wavelength 396 nm. In addition, SHG measurements show that 1 has nonlinear optical activity and its SHG response is 2.25 times that of standard potassium dihydrogen phosphate (KDP), which is promised with great potential to the development of new supramolecular fluorescence and nonlinear materials.